Use multiple bl_space_types to place the same toolbar on multiple locations
You can do this using a loop to generate dynamically named panels, each with the desired bl_space_type (based on this nice answer):

import bpy

def panelFactory( spaceType ):
    class NewClass( bpy.types.Panel ):
        bl_space_type = spaceType
        bl_region_type = 'TOOLS'
        bl_label       = "add_item_%s" % spaceType

        def draw( self, context ):
            layout = self.layout
            layout.operator( "mesh.primitive_cube_add" )

    NewClass.__name__ = "add_item_%s" % spaceType
    return NewClass

spaceTypes = ['VIEW_3D', 'IMAGE_EDITOR', 'NODE_EDITOR']

for spaceType in spaceTypes:
    panelFactory( spaceType )

def register():
    bpy.utils.register_module(__name__)

def unregister():
    bpy.utils.unregister_module(__name__)

if __name__ == "__main__":
    register()


This code will add the "Add Mesh Cube" operator in a new panel to the "Misc" tab in each of the desired windows (space types).